Matt, all i would say is if you can't weld or don't know someone who'll do it free, don't do it!!
welding can vary from £60 to £150 an hour, and there will be a fair amount of welding to do!!
I made a pit bike engined (20hp 150cc, 4spd manual) lawn mower 2 years ago for our annual rallyschool lawn mower race, was a bit bodged (to be kind), had the body of a ride on mower, engine over the front wheels to aid weight distribution, a quad axle and the steering off a 106 gti
all entirely home made and cost me about £400 (engine was £250), BUT, the steering rack was the wrong way round, so you turned left she went right, then wheeled it out the shed to load onto a trailer, pulled up the ramp, SNAP went the chain!! b****r. it's been sat in the back of the shed ever since. all she needs is a new steering mechanism bodging up, and a chain!!